Within the technique of packaging, non-returnable packages have been used for a long time which are manufactured from a packing laminate which consists of one or more carrier layers of paper or cardboard together with outer and inner coatings of thermoplastics, such as polyethylene. Frequently, the packing laminate in these so-called non-returnable packages is provided with a further layer of some other material, e.g. aluminium foil or plastic layers other than those mentioned here.
The composition of the packing laminate is designed to produce the best possible product protection for the goods which are to be packaged, as well as offering simple and convenient handling to the consumer. To achieve mechanical rigidity which, on the one hand, provides mechanical protection for the contents and, on the other hand, renders the package sufficiently form-stable so that it can be handled without any difficulty and gripped by hand, the packages of this type frequently are provided with a carrier layer of paper or cardboard which imparts rigidity of form and mechanical strength to the package. However, such a carrier layer possesses no tightness properties for gases or liquids, and the proper rigidity of the material disappears when the same is exposed to moisture or liquid which is absorbed into the material. In order to impart good liquid-tightness to the material, it is frequently laminated with a plastic material, and if this is thermoplastic (e.g. polyethylene) it may be used moreover, for sealing plastic layers to one another with the help of heat and pressure (so-called hot-sealing) and in this manner the packing container can be sealed and made permanent in its given form in that overlapping material panels, which are thermoplastic-coated, are heat-sealed to each other in a tight and mechanically strong seal.
Packing containers of the type mentioned here are usually manufactured either from blanks punched out beforehand or from a continuous web which has been prepared with suitable decoration of advertising and/or informative character and with a pattern of crease lines facilitating the fold-forming. Packing containers are made from such a web in that longitudinal edges of the web are joined to each other in an overlap joint so as to form a tube which subsequently is filled with the intended contents and is divided into individual, closed packing containers through repeated transverse sealings of the tube at right angles to the longitudinal axis of the tube. After suitable fold-forming of the packing material in the tube the material in the said packing containers is converted to the desired geometrical form, usually a parallelepiped, by providing the tube with longitudinal folding lines and with double-walled triangular folding lugs at the corners of the packing container. As an example of a known parallelepipedic packing container may be mentioned Tetra Brik (registered trademark) for the packaging of primarily liquid or semiliquid dairy products, e.g. milk, but also other types of beverages such as juice.
In view of the very strong growing market for non-returnable packing containers, there has been an unavoidable call (not least from the side of the consumer) for providing the packing containers with some type of visual identification mark in the form, for example, of colored patterns with some associated text and/or figures in order to distinguish a certain container from "rather similar" other containers and also for a clear indication of quality, date of packaging, keeping property etc. of the packed product. Such an advertising and/or information marking of packing containers is realized mainly in connection with the manufacture of the packing laminate and may be done, for example, in that a laminate web of continuous packing container blanks, plastics-coated on both sides and provided with crease lines, is introduced between and through the nip of rollers rotating in pairs close to each other, whereof one serves as an inking roller and consequently applies printing ink onto one side of the web according to a pattern corresponding to the desired marking on the finished package while the other roller in each such pair of rollers acts as a counter-pressure roller against the other side of the web passing by in accordance with conventional printing technique, whereupon the printed web is either rolled up into finished rolls or is divided up into individual packing container blanks for further transport to a packing machine for forming, filling and closing of the packing containers. Since this printing of decorations or patterns in printing plants usually takes place at very high production capacities, it is frequently an essential requirement that the printing ink composition used should be of a type to make possible a production-adapted rapid drying or hardening of the web before the web is rolled up or divided up into individual container blanks. This requirement is met satisfactorily by the known EB-hardenable (electron beam) printing ink compositions which contain monomers and/or oligomers hardenable by electron irradiation, and in the case of which a practically instantaneous "drying" (hardening) is possible by exposing the printed side of the web to electron irradiation of sufficient intensity. Among the merits of the method may also be counted the exceptionally good wear resistance of the hardened color layer which is obtained and which is a result of the hardening reaction initiated through electron irradiation of the printing ink composition applied. However such a printing method with electron irradiation in the form it is carried out at present is not entirely meritorious, but in fact has disadvantages too which, at least in parts, are assumed to be due to undesirable irradiation effects and which as a further consequence bring about impaired tightness properties of packing containers manufactured from such a decoration-printed packing laminate. Without being committed to any particular scientific theory, it is reasonable to assume that the irradiating electrons, in order to initiate the hardening action, must have allocated to them such energy that they thereby become sufficiently energetic to penetrate through the outer irradiated plastic layer and down into the underlying fiber layer, with a certain molecular degradation and/or structural alteration in that layer as a consequence. Thus, it has been found that moisture-repellent properties of a paper layer included in a known packing laminate so irradiated are drastically impaired, and that, as a result thereof, the packing laminate has become more moisture-sensitive and, consequently, less suitable for the manufacture of packing containers for liquid contents.